1. Field of the Invention
The present invention relates to an active noise control system which reduces overall noise level by outputting from an output transducer a canceling signal having a phase opposite to and the same amplitude as that of the noise, more particularly to an active noise control system for on-line secondary path modeling.
The invention arose during continuing development efforts relating to the subject matter shown and described in U.S. Pat. Nos. 4,677,676, 5,206,911, 5,390,255, 5,502,869, 5,553,153, 5,621,803 and 5,940,519, incorporated herein by reference.
2. Background and Summary
Active noise control systems have recently been applied to reduce noise produced by an air conditioner, an engine, a motor and traffics or the like. An active noise control system involves injecting a canceling signal having the same amplitude as that of the noise and a phase opposite to the noise so as to destructively interfere with and thus cancel an input noise. In a known active noise control system an output signal is sensed by an error transducer such as a microphone which supplies an error signal to a control model which in turn supplies a secondary signal to an output transducer such as a loudspeaker which injects a canceling signal to destructively interfere with and cancel an input noise. A digital signal processor (DSP), being a conventional noise controller, uses an adaptive filter of the finite impulse response (FIR) type which forms a signal for canceling noise upon receiving a reference signal from an input transducer such as a microphone, detects said error signal created by said error transducer such as a microphone. Said error signal is called residual noise and it is the result of cancellation. In other words: Said error signal has two functions: on the one hand it serves as a control signal for controlling the whole active noise control system by being fed to said elements described above and by being fed to elements still to be described. With respect to this function it is called “error signal”. On the other hand said error signal is, at the same time, the output signal of the whole active noise control system. With respect to this function it is called “residual noise”. So, whenever in this specification and in the appended claims either the expression “error signal” or the expression “residual noise” are used, they stand for one and the same signal. Said digital signal processor performs a feedback control using a reference signal and said error signal. In this feedback control, furthermore the level of said error signal can be minimized by controlling the filter coefficients of said adaptive filter. Said adaptive filter may use any of a variety of known and available adaptive algorithms, such as the so-called least-mean-square (LMS) algorithm. Detailed descriptions of adaptive algorithms are given in “Adaptive Signal Processing” by B. Widrow and S. D. Stearns, Prentice Hall, (1985).
U.S. Pat. No. 4,677,676 by L. J. Eriksson discloses an active noise control system. This system is a typical example of the prior art. An auxiliary noise source is used to model feedback and secondary paths. The auxiliary noise source is random and uncorrelated to an input noise. The operation of an adaptive filter of an active noise controller affects the on-line path modeling because both the input noise and the canceling signal are the disturbances for the modeling. Furthermore, the auxiliary noise source increases the residual noise in two aspects. Firstly the auxiliary noise contributes to the residual noise through the output transducer. Secondly the auxiliary noise is a perturbation to the operation of the controller adaptive filter, thus the residual noise increases due to degraded performance of the controller.
U.S. Pat No. 5,553,153 by G. P. Eatwell uses a fixed auxiliary signal for on-line secondary path modeling, which requires less computation and reduces coefficient jitter in the adaptive filter. Again, the operation of the active noise controller still affects the on-line path modeling and the auxiliary noise source still increases the residual noise due to addition of the auxiliary noise itself to the residual noise.
U.S. Pat. No. 5,940,519 by S. M. Kuo describes a feedforward active noise control system which performs on-line feedback path modeling and on-line secondary path modeling. Although the disturbances from both the input noise and the canceling signal for the on-line modeling are reduced, the auxiliary noise source still increases the residual noise in both two aspects as stated above.